1. Field of the Invention
The present invention relates to a variable steering ratio steering system capable of varying a steering ratio of steering wheel rotation angle to steer angle at road wheels.
2. Description of the Prior Art
Many steering systems provide variable-ratio steering. One such variable steering ratio steering system has been disclosed in Japanese Patent Provisional Publication No. 4-243667. In the variable-ratio steering system disclosed in the Japanese Patent Provisional Publication No. 4-243667, in a similar manner as a typical recirculating-ball steering gear, a ball-screw mechanism is constructed by a worm shaft on the end of the steering shaft, a ball nut, and recirculating balls interposed between the worm teeth of the worm shaft and grooves cut in the inner face of the ball nut. A rotating member is mechanically linked to the steered wheel and interleaved between the steered wheel (the road wheel) and the steering wheel, so that the rotating member rotates according to the steering of the road wheels. A converting nut member is fitted oscillatingly onto the ball nut and housed so that the converting nut member is axially movable and rotatable about the axis. The converting nut member is also engaged with a lever member. By virtue of engagement between the converting nut member and the lever member, the oscillating motion (rotational displacement) of the converting nut member produces a change in the distance from the rotation center of the rotating member to the engaged position (the engaged point) between the tip of the armed portion of the converting nut member and the inner peripheral wall surface of the lever member. Also provided is a converter which permits a rectilinear motion of the converting nut member and a pivotal motion of the lever member to be transmitted in the mutually convertible state. The converting nut member is driven by means of a drive mechanism so as to produce the previously-noted oscillating motion (that is, the change in the distance from the rotation center of the rotating member to the engaged position). Depending on the change in distance from the rotation center of the rotating member to the engaged position, it is possible to vary the ratio of the pivotal angle of the lever member to the displacement of the converting nut member, that is, the steering ratio. As discussed above, in the steering system disclosed in the Japanese Patent Provisional Publication No. 4-243667, a rotary motion of the steering wheel is converted into a linear motion acting in the axial direction of the steering shaft by means of the ball screw mechanism in a conventional manner, and then the linear motion of the steering shaft is converted into rotational displacement (pivotal movement) by means of the lever member cooperating with the converting nut member. The rotational displacement (pivotal movement) of the lever member is further converted into a linear motion of a rack in the longitudinal direction of the rack by means of a rack-and-pinion steering gear mechanism.
The previously-discussed conventional variable ratio steering system is complicated in structure. The steering-wheel rotation angle, that is, the rotational angle (or torque) of the steering wheel, is transmitted to the rack through the steering shaft, the ball nut, the converting nut member, the lever member, and the rack-and-pinion steering gear mechanism. Such a complicated variable-ratio steering system increases the number of parts of the system as well as the total weight of the system. Additionally, bearings, gears, or the like must be provided between parts coupled to each other. Many component parts results in many coupling portions, thus increasing friction loss (energy loss), and resulting in undesired rattle or freeplay (noise).
Accordingly, it is an object of the invention to provide a variable steering ratio steering system which is simple in construction while providing variable-ratio steering.
In order to accomplish the aforementioned and other objects of the present invention, a variable-ratio steering system comprises a steering shaft adapted to be connected to a manually-operated handle, an intermediate member engaging with the steering shaft, and capable of moving in a direction of an axis of the steering shaft by a rotary motion of the steering shaft, a first converter converting the rotary motion of the steering shaft into an axial displacement of the intermediate member in the direction of an axis of the steering shaft, a second converter generating a rotational displacement of the intermediate member about the axis of the steering shaft with the axial displacement of the intermediate member, a pinion gear connected to the intermediate member and capable of rotating together with the intermediate member, a rack engaged with the pinion gear, and adapted to be connected at both ends of the rack via knuckle mechanisms to steered wheels, for varying a steer angle at the steered wheels depending on a displacement of the rack, and the second converter having a variable steering ratio mechanism which varies a ratio of a rotational displacement of the intermediate member about the axis of the steering shaft to the axial displacement of the intermediate member.
According to another aspect of the invention, a variable-ratio steering system comprises a steering shaft adapted to be connected to a manually-operated handle and having a worm gear portion, a ball nut engaging with the worm gear portion of the steering shaft via recirculating balls, and capable of moving in a direction of an axis of the steering shaft by a rotary motion of the steering shaft, a first converting means for converting the rotary motion of the steering shaft into an axial displacement of the ball nut in the direction of an axis of the steering shaft, a second converting means for generating a rotational displacement of the ball nut about the axis of the steering shaft with the axial displacement of the ball nut, a pinion gear block connected to the ball nut for rotating together with the ball nut, a rack engaged with the pinion gear block, and adapted to be connected at both ends of the rack via knuckle mechanisms to steered wheels, for varying a steer angle at the steered wheels depending on a displacement of the rack, and the second converting means having a variable steering ratio mechanism which varies a ratio of a rotational displacement of the ball nut about the axis of the steering shaft to the axial displacement of the ball nut to vary a steering ratio of steering wheel rotation angle to steer angle at the steered wheels.